The present invention relates to an electron beam apparatus equipped with two deflection coils located between a condenser lens and an objective lens. Designed for use on scanning electron microscopes, etc., this electron beam apparatus employs the deflection coils so as to deflect the beam and to permit scanning of the specimen surface. There has been a need to scan the specimen over a wide variety of speeds, ranging from a slow scanning speed to a television scanning speed. To meet such a requirement, the conventional electron beam apparatus illustrated in FIG. 1 was constructed.
In FIG. 1, an electron beam 2 emitted from an electron gun 1 is condensed onto an aperture plate 4 by a condenser lens 3. The electron beam that passes through an opening 5 of the plate 4 is deflected by an upper toroidal coil 6 at an angle .theta..sub.1 and deflected back by a lower toroidal coil 7 at an angle .theta..sub.2, and passes through the center of an objective lens 8. Thus, the electron beam 2 is deflected at a total angle of .theta.=.theta..sub.2 -.theta..sub.1 with respect to an optical axis 9 before it reaches a specimen 10. A television scanning pulse generating circuit 11 produces an output signal which is supplied via a magnification control circuit 12 to terminals (a) of switches 13, 14 which are connected via amplifiers 15, 16, respectively, to the upper and lower toroidal coils 6, 7. A sawtooth generator 17 produces scanning signals having different scanning speeds ranging from a low scanning speed to the so-called super rapid scanning speed (which is slightly slower than a television scanning speed), the output signal from the sawtooth generator 17 being supplied via a magnification control circuit 18 to terminals (b) of the switches 13, 14. A secondary electron detector 19 produces a detection signal which is supplied to a cathode ray tube (not shown), which is scanned in synchronization with the scanning of the specimen by the electron beam.
When it becomes necessary to change the scanning speed of the electron beam 2 from a low scanning speed to a super rapid scanning speed in a conventional electron beam apparatus, the switches 13, 14 are connected to the terminals (b) to allow the output signal from the sawtooth generator 17 to be supplied to the upper and lower toroidal deflection coils 6, 7, and the sawtooth generator 17 is adjusted to generate scanning signals of the desired frequencies. On the other hand, for effecting scanning at a television scanning speed, the switches 13, 14 are connected to the terminals (a) to allow square-wave pulses from the pulse generator 11 to be supplied to the deflection coils 6, 7. (The square-wave pulses are distorted in waveforms by the deflection coils 6, 7 in which there actually flows current of a sawtooth waveform.)
Such switching between the scanning signal sources is necessary for the following reasons:
In general, the upper deflection coil must be small in size, since it is necessary that the upper deflection coil be located adjacent to the aperture plate. Therefore, the upper deflection coil is selected as a toroidal coil. However, in the conventional apparatus, the lower deflection coil is also selected as a toroidal coil. While toroidal coils are small in size, they have a large self-inductance (hereinafter called "inductance") per unit deflection angle. Therefore, the upper and lower toroidal deflection coils 6, 7 have a comparatively large combined inductance. Owing to the large inductance, when a current that approximates a television scanning frequency is supplied from the sawtooth generator in order to satisfy the need for high-speed scanning observation, the sawtooth waveform of the current becomes distorted while flowing through the deflection coils, resulting in poor scanning operation. Therefore, switching is necessary to allow the pulse generator 11 to supply current of a square waveform, which will be distorted into a sawtooth waveform, for television scanning. However, it is difficult to keep the zero points of the pulse generator 11 and the sawtooth generator 17 in line with each other at all times. Therefore, the specimen image shifts, in effect preventing observation of the same region of the specimen image when changing scanning speeds between television scanning speeds and other scanning speeds.
Moreover, the use of two sources of scanning signals renders the apparatus complex in structure and costly to manufacture.